The present invention relates to a vehicular drive apparatus.
A typical automatic transmission mounted in a vehicle or a typical hybrid drive system mounted in a hybrid vehicle includes a plurality of clutches and brakes that apply and release based on an application pressure supplied to a hydraulic servo in a case of the apparatus. In these apparatuses, the transmission path of a driving force from an internal combustion engine (hereinafter simply referred to as “engine”) or an electric motor through a planetary gear unit is changed according to the timely operation of the clutches and brakes. As a result, the rotation speed and torque are changed as they are transmitted to driven wheels at the end of the transmission path.
One known automatic transmission having such a configuration has a cutout or an open portion formed in a peripheral surface of a piston member of a hydraulic servo of the automatic transmission. The cut out and the open portion are for inserting a speed detecting rotation sensor from outside the case (see Japanese Patent Application Publication No. JP-A-05-172222, for example).
In the automatic transmission disclosed in Japanese Patent Application Publication No. JP-A-05-172222, when the piston member pushes against a set of a plurality of friction plates sequentially arranged in the axial direction inside the case, the piston load increases at the two end portions of the open portion in the peripheral surface of the piston member. This may affect the surface pressure distribution when the piston member pushes against the friction plate set. In the automatic transmission disclosed in Japanese Patent Application Publication No. JP-A-05-172222, however, because a separate brake is not arranged to race the friction plate that is set to be pushed by the piston member described above, no spline grooves are located on the opposite side of a backup plate of the friction plate set from the piston. As a result, the effect on the surface pressure distribution will not become worse when the piston member pushes against the friction plates.
In contrast to this, there exists an automatic transmission that has a brake structure with friction plate sets arranged opposing one another in the axial direction inside a case. The friction plate set of one brake is received by a snap-ring and the friction plate set of the other brake is received by a backup plate (see Japanese Patent Application Publication No. JP-A-09-280328, for example).
With a structure such as that of the automatic transmission disclosed in Japanese Patent Application Publication No. JP-A-09-280328, however, for example, when the friction plates of the other brake are being pushed, the back surface of the backup plate of the other brake (that is to be supported by the case surface) is not able to be supported evenly. This occurs because there are spline grooves for spline-engaging the friction plates of the one brake on a side of the back surface of the friction plates of the other brake that are being pushed. For example, when considering this with the structure disclosed in Japanese Patent Application Publication No. JP-A-05-172222, when the friction plate set having the backup plate where the back surface is not evenly supported is pressed against by the piston member having the cutout or open portion in the peripheral surface, there may be a significantly adverse effect on the surface pressure distribution at the area of the backup plate that is being pressed against by the portion corresponding to the cutout or open portion of the piston member.
That is, the presence of the open portion of the piston member reduces the rigidity of the portion corresponding to that open portion, but the rigidity of the other portions does not change. Accordingly, the amount of deformation of the open portion changes with respect to the portions other than the open portion such that there is a difference in deformation amount. In particular, the boundary portion between a portion with low rigidity and a portion with high rigidity deforms. At this time, the stress acting on the deformed portion increases and causes the deformation state to change compared with other portions. As a result, problems such as the friction material peeling away from the disc may occur due to deterioration of the surface pressure distribution of the piston member.